The primary aim of the Health and Retirement Study (HRS) cooperative agreement is to collect and distribute multi-disciplinary data to support research on aging. Cognitive impairment and dementia, for which Alzheimer's disease is the principal cause either alone or in combination with other factors, impose enormous burdens on families and government policies. Population-based studies can complement clinical studies to enhance understanding of the etiology and impact of dementia. With consistent measurement, trends over time and variation across countries can further expand our knowledge. This revision application, in response to NIA notice NOT-AG-14-022, aims to build on prior work in the HRS and its previous dementia sub-study, the Aging, Demographics, and Memory Study (ADAMS) to implement a cost-effective algorithmic approach to assessing cognitive impairment and dementia. This new Harmonized Cognitive Assessment Protocol (HCAP) will provide a basis for comparing trends over time in the US and for comparison across countries with HRS-like longitudinal studies of aging. The proposed sample of 3,000 persons aged 65 and older drawn from the HRS will create the potential for longitudinal follow-up to observe trajectories of decline and study their correlates in the rich data collected by HRS. We also propose to collect and store samples of venous blood from HCAP participants for future use.